Likely you have adjusted the tempo. By default MusE has the Master track enabled which allows for changing tempo during the song.
You can inspect the master track with two editors Graphic and List, they are available on the Edit menu under Master track.
If you are not interested in making tempo changes, and admittedly I never do , bring up the transport (F11 by default) and click the button [Master] and the master track will be disabled. Now when you change the tempo it will affect the entire song, not from the position where you changed it.

Likely you have adjusted the tempo. By default MusE has the Master track enabled which allows for changing tempo during the song.
You can inspect the master track with two editors Graphic and List, they are available on the Edit menu under Master track.
If you are not interested in making tempo changes, and admittedly I never do , bring up the transport (F11 by default) and click the button [Master] and the master track will be disabled. Now when you change the tempo it will affect the entire song, not from the position where you changed it.